When a fuel vaporizes in an intake system, it has a cooling effect, and it just so happens that ethanol cools much more than gasoline. The cooling effect is referred to as the latent heat of vaporization and is expressed as Btu per pound. For ethanol, the number is 396 Btu per pound, and for gasoline that number is approximately 150 Btu per pound. As one can see, this is where the advantage is.
Marvin Benoit: In addition to the octane advantage of E85 over gasoline, since it’s an alcohol-type fuel, most motors will generally produce more torque on E85. In fact, we have local NHRA Super Street racers who have had great success running E85. For bracket racers, the lower cost of E85 makes it a great deal. Furthermore, motors will typically run 15-20 degrees cooler on E85 than gasoline. In some cases, that can be enough to solve overheating problems. Ultimately, you can put a bigger motor in a street car with E85 without having to worry about overheating, and have more horsepower thanks to the cooling effect of the alcohol content.
E85 requires a richer air/fuel mixture than gasoline because, ideally, we want to burn as much fuel as we can with the amount of oxygen in the air. With E85, additional oxygen is contained in the fuel itself so the mixture must be richer to burn all of the fuel and oxygen available. Pure ethanol contains 34.7 percent oxygen compared to most gasolines that contain between 0 and 3.5 percent. This means that more fuel can be burned when using greater amounts of ethanol. The reason that power does not continue to increase is because ethanol is much lower in energy content than gasoline. The Brake Specific Fuel Consumption numbers for E85 will be much higher than for gasoline because more pounds of ethanol are required to make comparable power numbers to gasoline. If you attempted to operate a carbureted engine on E85 without richening the mixture, the air/fuel ratio would be very lean and the engine would misfire along with having very poor driveability. Additionally, E85 contains approximately 82,000 Btu per gallon and pure gasoline with no ethanol in it contains approximately 115,000 Btu per gallon. This is why the air/fuel ratio for an E85 fuel must be much richer than for gasoline.
In many naturally aspirated street engines, E85 and gasoline will produce about the same horsepower. That said, many hot rodders have built forced-induction motors that run quite well on E85. In a forced-induction system, E85 is better than 91-octane street gasoline on a cost basis because of the increased cooling effect it provides in the induction system. Richer air/fuel mixtures contribute to a greater amount of cooling, which enhances power because it enhances volumetric efficiency. The estimated octane number for street blends of E85 is 105 using the (R+M)÷2 octane rating method. These numbers are optimistic because the quality of the 15 percent gasoline portion can vary significantly. Rockett Brand Racing E85 fuel will work much better than street E85 available at regular gas pumps because of its superior consistency from batch to batch. It’s important to note that at high boost levels, E85 will run out of octane quality to satisfy an engine’s operating conditions. That is where a high-quality, high-octane gasoline will outperform an E85 fuel.